Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich (see for example Japanese Patent Publication (A) No. 2004-108176). In this internal combustion engine, if the NOx storage ability of the NOx storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich whereby NOx is released from the NOx storing catalyst and reduced.
In this way, in the past, when releasing NOx from an NOx storing catalyst, the air-fuel ratio of the NOx exhaust gas is made the stoichiometric air-fuel ratio or rich to release the NOx which is then reduced in the NOx storing catalyst. However, depending on the case, there are also cases when it is preferable for the NOx released from the NOx storing catalyst to be exhausted from the NOx storing catalyst without being reduced.